In case where an air bag is provided for protecting an occupant sitting on a vehicle seat, an occupant determination device is provided at the vehicle seat for determining whether an occupant is seated or not on the corresponding seat, the occupant is an adult or a child, or the like. Japanese Patent Laid-open Publication No. 09-207638 discloses a known occupant determination device. The disclosed device includes a plurality of load sensors provided at a plurality of fixing positions respectively of a seat body facing a vehicle floor, and a controller for calculating a detection load value based on load values output from the plurality of load sensors and for determining whether or not an occupant is seated on the vehicle seat based on the calculated detection load value. More specifically, the controller calculates a detection load value by summing up the load values output from the plurality of load sensors using an adder, compares this detection load value with a load value set in advance (threshold value for determination) using a determination processing circuit, and determines whether or not an occupant is seated on the vehicle seat by comparing the detection load value and the threshold value for determination.
When the occupant determination to determine a child or an adult is made based on the difference between a detection load value and a determination threshold value, if the detection load value corresponding to a weight of an adult is close to the determination threshold value, the changing of the determination (from a child to an adult or vice versa) may be frequently made due to the vibration of the vehicle or the change of the occupant posture. In order to prevent such frequent changes of determination, a delay time is provided at changing the determination. However, since the vibration of the vehicle or the change of the posture of the occupant may occur not periodically or may continue for not constant time, the delay time setting is not a sufficient solution for the problems. In another known art, Japanese patent Application published as 11-198705, a proposal has been made to use an output pattern of an antenna embedded in the seat back to improve the accuracy of the occupant determination. In further known art, Japanese Patent Application published as 2000-127890, the device disclosed in this art detects a seatbelt tension and corrects the determination threshold value based on the tension value to improve the accuracy of the occupant determination. However, in such known art method, it is necessary to provide a detecting element for exclusive use which may lead to cost increase.